Plzz solve ..thank u

Question 4 (10 marks) In order to complete a certain building, a contractor has developed the following PERT network to help in analyzing the project. The (Ti, Vi) values refer to the mean duration (in weeks), Ti, and variance, Vi, for PERT activities. Activity A B C D E F G H Preceding A A A B B.C D G E.F.H activities (Ti, Vi) (2,2) (4,3) (5,1) (7,5) (6,2) (9,5) (4,1) (8,3) (3,1) values The contractor wants to find